Thin films of metal oxides (e.g., titanium dioxide) have been used in the manufacture of high temperature optical components such as band pass filters. Such films can be deposited by chemical vapor deposition methods such as are described in, for example, U.S. Pat. Nos. 4,006,481 and 4,211,803.
While such films in the as-deposited state do not exhibit scattering at ambient or slightly elevated temperatures, scattering does take place after use at elevated temperatures adversely affecting optical performance. It is therefore desirable to prevent the development of optical scattering after use at elevated temperatures in optical elements used for the production of infrared mirrors for incandescent lamps and other optical elements which benefit from low scattering under usual use conditions.
Applicants' have discovered that metal oxide films which do not scatter radiation in the as-deposited state develop a tendency to scatter after high temperature use and that overcoating of the metal oxide film with a layer of amorphous silicon dioxide preserves the non-scattering character of the as-deposited metal oxide layer.
In a preferred embodiment of the invention, the metal oxide is titanium dioxide which is preferably deposited by the chemical vapor deposition from titanium tetraethoxide. In another preferred feature of the invention, the metal oxide film and the silicon dioxide layer are deposited by use of an effusion cell arranged in flow communication with the deposition chamber to enable blending of the metal oxide and the silicon dioxide at their interface in order to provide improved adhesion and reduce stress at the interface by avoiding an abrupt interface.
It is therefore an object of the invention to provide an amorphous silicon dioxide coated metal oxide film which is substantially free of optical scattering at elevated temperatures.
One of the objects of this invention is to provide optically nonscattering films of titanium dioxide.
Another object of the invention is to provide a method for preventing the development of a radiation scattering microstructure in chemical vapor deposited titanium dioxide films.
Still another object of the invention is to provide a method for making non-scattering composites comprising titanium dioxide and silicon dioxide layers.